The invention relates to arranging redundancy in a process control system.
In an industrial process, a highly automatized system is used for ensuring that the process operates in a correct manner. An industrial process, such as a paper making process or a power station process, is very extensive and complex, including many variables. An information system used for controlling an industrial process is responsible for various tasks relating to collecting, distributing, storing and presenting process properties as well as to process control. A process control system typically comprises a large number of work stations that operate independently or controlled by an operator. Such work stations carry out different process control related tasks, such as processing of measurement data and storage of historical data, according to the applications to be executed in the work stations.
Conventionally, process control systems have employed proprietary hardware and software solutions, in which case devices from different manufacturers are unable to communicate with each other. In such a case, it has been necessary to determine proprietary interfaces for applications in order to access data of different devices. In the process control industry, several standards have thus been developed to determine open communication interfaces to be used between the different devices of a process control system. An example of such an open standard is OPC (Object Linking and Embedding for Process Control). The OPC provides a set of interfaces, properties and methods based on ActiveX/COM (Component Object Model) technologies to be used for process control applications. A bus solution supporting the OPC standard comprises OPC clients and OPC servers. The OPC servers may communicate with proprietary devices and transfer data to different OPC clients that forward data to applications utilizing the data. The OPC enables a common interface to be provided for the applications so as to enable access to the data of different process control devices.
The functionality of OPC servers serving up to dozens of OPC clients thus plays a crucial role in the information system. In order to ensure the functionality of a system, redundancy of OPC servers may thus be arranged by doubling, meaning that in addition to a primary OPC server, the system is also provided with a secondary OPC server. Such a redundancy arrangement has been disclosed in WO 0 023 857, the solution according to the publication comprising switching over to using the secondary OPC server when the primary OPC server is no longer available. The secondary server may start carrying out the tasks of the primary server without the client devices noticing the change (the logical connection provided for the client device remains unchanged although the server device providing the connection changes). The solution disclosed in WO 0 023 857 does not, however, take any other error situations but server or connection failure into account, thus providing limited redundancy. Even if a server were working correctly, the connection e.g. between the server and a field or another device connected thereto may be completely lost or otherwise subjected to interference.